This invention pertains generally to the control of core operation of a nuclear reactor and more particularly to the control of the axial power distribution within the core.
Generally nuclear reactors contain a reactive region commonly referred to as the core in which sustained fission reactions occur to generate heat. The core includes a plurality of elongated fuel rods comprising fissile material, positioned in assemblies and arranged in a prescribed geometry governed by the physics of the nuclear reaction. Neutrons bombarding the fissile material promote the fissionable reaction which in turn releases additional neutrons to maintain a sustained process. The heat generated in the core is carried away by a cooling medium, which circulates among the fuel assemblies and is conveyed to heat exchangers which in turn produce steam for the production of electricity.
Commonly in pressurized water reactors a neutron absorbing element is included within the cooling medium (which also functions as a moderator) in controlled variable concentrations to modify the reactivity and thus the heat generated within the core, when required. In addition, control rods are interspersed among the fuel assemblies, longitudinally movable axially within the core, to control the cores reactivity and thus its power output. There are three types of control rods that are employed for various purposes. Full length rods, which extend in length to at least the axial height of the core, are normally employed for reactivity control. Part length control rods, which have an axial length substantially less than the height of the core, are normally used for axial power distribution control. In addition, reactor shutdown control rods are provided for ceasing the sustained fissionable reaction within the core and shutting down the reactor. The part length rods and full length control rods are arranged to be incrementally movable into and out of the core to obtain the degree of control desired.
As a byproduct of the fissionable reaction, through a process of beta decay of radioactive iodine, xenon is created. Xenon has the property of having a large neutron absorption cross section and therefore has a significant effect on the power distribution within the core and reactivity control. While the other forms of reactivity management are directly responsive to control, the xenon concentration within the core creates serious problems in reactor control in that it exhibits a relatively long delay period and requires up to at least twenty hours after a power change to reach a steady-state value.
While the radial power distribution of the core is fairly uniform, due to the prescribed arrangement of fuel assemblies and the positioning of control rods which are symmetrically situated radially throughout the core, the axial power distribution can vary greatly during reactor operation. Core axial power distribution has created many problems throughout the history of reactor operations for many reasons. Normally coolant flow through the fuel assemblies is directed from a lower portion of the core to the upper core regions, resulting in a temperature gradient axially along the core. Changes in the rate of the fissionable reaction, which is temperature dependent, will thus vary the axial power distribution. Secondly, the axial variation in the power distribution varies the xenon axial distribution, which further accentuates the variations in power axially along the core. Thirdly, insertion of the control rods from the top of the core, without proper consideration of the past operating history of the reactor can add to the axial power asymmetry.
The change in reactor core power output which is required to accommodate a change in electrical output of an electrical generating plant is commonly referred to as load follow. One load follow control program currently recommended by reactor vendors utilizes the movement of the full length control rods for power level increases and decreases and the part length control rods to control axial oscillations and shape the axial power profile. Changes in reactivity associated with changes in the xenon concentration are generally compensated for by corresponding changes in the concentration of the neutron absorbing element within core coolant or moderator. In this mode of operation the part length rods are moved to maintain the axial offset within some required band, typically plus or minus 15%. The axial offset is a useful parameter for measuring the axial power distribution and is defined as: EQU A.O. = (P.sub.t -P.sub.b)/(P.sub.t +P.sub.b)
where P.sub.t and P.sub.b denotes the fraction of power generated in the top half and the bottom half of the core respectively. No effort is made to maintain the inherent core axial power profile. The part length rods are moved to minimize and reduce the axial offset independent of the previously established steady-state axial offset. This process induces a constant fluctuation of the axial offset during sustained load follow operations which result in a number of undesirable operating conditions. For one thing power pinching, which is a large axially centered power peak, is likely to occur. Such power peaks result in a reactor power penalty which requires the reactor to be operated at a reduced level so that such peaks do not exceed specified magnitudes. Secondly, severe changes occur in the axial power profile of a transient nature during large load changes due to heavy insertion of control rods at reduced power levels. Thirdly, large xenon transients occur upon coming back to power resulting in occurrences such as axial power oscillations. Fourthly, the part length rod broad operating instructions are generally vague and require anticipation and interpretation by the reactor plant operator. Fifthly, increased hot channel factors result (which are hot spots which occur within the cooling channels among the fuel assemblies) and require reductions in the power rating of the reactor to accommodate these severe transients and/or adverse power profiles. Finally, no protection currently exists against severe pinching with small axial offsets.
Due to the many adverse operating conditions experienced in operating a nuclear reactor during load follow many reactor vendors recommend operating the reactor at a constant power output without a load follow capability.
Accordingly, a new method of operating a nuclear reactor is desired that will have a load follow capability without exhibiting the adverse operating conditions described above, thereby avoiding the necessity of imposing power penalties to compensate for axial power peaks.